Okay i made it work with a variable, it was pretty easy. The variable accepted multiline text easily.
Thanks!

databricks.yml

targets:
  dev:
    mode: development
	...
    variables:
      notifications:
        default: 
          - user1@smth.com
          - user2@smth.com

job:

resources:
  jobs:
    Master_Load:
      name: Master Load
      email_notifications:
        on_failure:
          ${var.notifications}
        on_duration_warning_threshold_exceeded:
          ${var.notifications}